code: def aggregate(self, results):
      for result in results:
          assert 2 == result
      return sum(results)
  
file_path: chainer/tests/chainermn_tests/extensions_tests/test_multi_node_evaluator.py
imports: import chainer
  import chainer.testing
  from chainer.datasets import TupleDataset
  from chainer.iterators import SerialIterator
  from chainermn import create_communicator
  from chainermn.extensions import GenericMultiNodeEvaluator
label_desc: Use image node_id 3 for calling the CustomMultiNodeEvaluator obj's underlying member method code with example usage: obj.aggregate(results) and returns: sum